linux-phytec-ti

rbd: plug rbd_dev->header.object_prefix memory leak

Need to free object_prefix when rbd_dev_v2_snap_context() fails, but
only if this is the first time we are reading in the header.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>